The Marvel Cinematic Universe (MCU) has introduced a lot of reality-bending storylines over the years. From Captain America surviving for over 50 years frozen in ice to Thor traveling across worlds and Loki possessing innocent scientists, the MCU defies laws of nature at every turn. And yet, there is one facet of life that Marvel has not dared play with until now: time. Specifically, time travel. Yes, Captain America and The Winter Soldier are living out of their time, but they did not travel through time. But, with Doctor Strange, the MCU is finally dipping a toe in that world. Spoiler alert — the Doctor Strange ending opens up the door for time travel in the MCU going forward, which could have immense repercussions when it comes to the Avengers.
In the end of Doctor Strange, Strange uses the Eye of Agamotto and the Time Stone to turn back the clock — literally. In doing so, he saves the planet from the evil Dormammu, who wants to consume the Earth into the Dark Dimension, ensuring that the Earth, and its Avengers, will live to fight another day. The Avengers themselves are, thus far, unaware of Doctor Strange's journey through time, just as they are of Doctor Strange, but just because nobody knows about it doesn't mean that time travel, specifically as shown in Doctor Strange, won't come into play in future Marvel movies, specifically the upcoming Avengers films.
In Doctor Strange, the Eye of Agamotto is established as as a very dangerous tool for sorcerers, not to be used with any kind of regularity — a fact made clear by Doctor Strange removing the Eye from around his neck at the end of the film. In other words: time travel is probably not going to be a plot device used to defeat every single Marvel villain, nor will it be used to reunite Captain America and Agent Carter as much as we wish it would be. However, it seems increasingly likely that time travel or time manipulation, as introduced in Doctor Strange, will be featured in the final two Avengers film.
The Doctor Strange ending establishes the power of the Time Stone, which is, as any Marvel fan should know, one of the six Infinity Stones. As an Infinity Stone, it will be sought after by Thanos, the Avengers villain first teased in the first end-credits scene of The Avengers. Thanos has been confirmed as the villain in Avengers: Infinity War, the third Avengers film scheduled for release in 2018, as has Doctor Strange. It's likely, then, that the Time Stone will be somewhat crucial to the plot (along with the other Infinity Stones seen throughout the MCU). But, Doctor Strange doesn't only open up the door to time travel in the MCU, it also opens up the multiverse.
When we meet Thanos in Infinity Wars, he will presumably be on the hunt to find all six Infinity Stones to power his Infinity Gauntlet, something he did in the comics, as noted by MoviePilot. The Gauntlet is one of the most powerful objects across the Marvel multiverse, meaning that the Time Stone and Doctor Strange will be dragged into the Avengers' fight against Thanos. Since the ending of Doctor Strange doesn't explicitly connect the Time Stone to Thanos, Doctor Strange ended the film completely unaware of the threat ahead. But Thanos will be coming for the Infinity Stone, whether Strange knows it or not.
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